Re: chunk-based cache in ceph with erasure coded back-end storage

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the data encode/decode operation is done on OSD side.

2016-03-31 23:28 GMT+08:00 Yu Xiang <hellomorning@xxxxxxxxxxxxx>:
> Thanks for the reply!
> So where did the decoding process happen? Is it in cache or on the client
> side? (Only considering Read.) If it happened when copying from storage tier
> to cache, then it has to be an whole object (file), but if decoding can be
> happened on client side when the client has all needed chunks, it seems
> cache can hold partial chunks of the file? What i mean is that is it
> possible for cache to hold partial chunks of a file in Ceph?  (assuming file
> A has 7 chunks in storage tier, to recover file A a client needs 4 chunks,
> will it be possible that 2 chunks of file A are copied to and stored in
> cache, when file A is requested, only another 2 chunks are needed from the
> storage tier? )
>
> Thanks!
>
>
>
> e-----Original Message-----
> From: huang jun <hjwsm1989@xxxxxxxxx>
> To: Yu Xiang <hellomorning@xxxxxxxxxxxxx>
> Cc: ceph-users <ceph-users@xxxxxxxxxxxxxx>
> Sent: Wed, Mar 30, 2016 9:04 pm
> Subject: Re:  chunk-based cache in ceph with erasure coded
> back-end storage
>
>
> if your cache-mode is write-back, which will cache the read object in
> cache tier.
> you can try the read-proxy mode, which will not cache the object.
> the read request send to primary OSD, and the primary osd collect the
> shards from base tier(in you case, is erasure code pool),
> you need to read at least k chunks to decode the object.
> In current code, cache tier only store the whole object, not the shards.
>
>
> 2016-03-31 6:10 GMT+08:00 Yu Xiang <hellomorning@xxxxxxxxxxxxx>:
>> Dear List,
>> I am exploring in ceph caching tier recently, considering a cache-tier
>> (replicated) and a back storage-tier (erasure-coded), so chunks are stored
>> in the OSDs in the erasure-coded storage tier, when a file has been
>> requested to read, usually, all chunks in the storage tier would be copied
>> to the cache tier, replicated, and stored in the OSDs in caching pool, but
>> i
>> was wondering would it be possible that if only partial chunks of the
>> requested file be copied to cache? or it has to be a complete file? for
>> example, a file using (7,4) erasure code (4 original chunks, 3 encoded
>> chunks), when read it might be 4 required chunks are copied to cache, and
>> i
>> was wondering if it's possible to copy only 2 out of 4 required chunks to
>> cache, and the users getting the other 2 chunks elsewhere (or assuming the
>> client already has 2 chunks, they only need another 2 from ceph)? can the
>> cache store partial chunks of a file?
>>
>> Thanks in advance for any help!
>>
>> Best,
>> Yu
>>
>> _______________________________________________
>> ceph-users mailing list
>> ceph-users@xxxxxxxxxxxxxx
>> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
>>
>
>
>
> --
> thanks
> huangjun



-- 
thanks
huangjun
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